Field of the Invention
The present invention concerns the field of magnetic resonance imaging, and in particular to magnetic resonance imaging techniques wherein magnetic resonance signals are acquired simultaneously from multiple slices of a subject.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency gives the excited spins a magnetization that causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The emitted MR signals have a signal intensity that is dependent on the exponential decay over time of the magnetization of the nuclear spins. The acquired signals are digitized so as to form raw data, which are entered into a memory that is organized as k-space, as k-space data. Many techniques are known for reconstructing an image of the examination object from the k-space data.
By appropriately selecting different characteristics of the MR data acquisition sequence that is used, the acquired signals can be differently weighted so that different sources of the detected MR signals (i.e., different tissues in the case of medical MR imaging) appear with different contrasts in the reconstructed image. In the case of medical MR imaging, a weighting is selected that causes the tissue that is important for making the intended medical diagnosis to have the best contrast (brightness) in the reconstructed image. One such type of weighting is known as T1-weighting, because it depends on the so-called T1 relaxation time of the nuclear spins.
Many different techniques are known for acquiring the raw MR data. One such technique is known as simultaneous multi-slice (SMS) acquisition, which is a technique for accelerating the acquisition of the data from a given volume of the examination object, wherein nuclear spins in multiple slices are excited simultaneously, and the resulting MR signals are simultaneously acquired from each slice. This results in a dataset in k-space that is composed of data from the multiple slices collapsed on top of each other. Techniques are known for separating or uncollapsing the data for these respective slices during image reconstruction, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, which is schematically illustrated in FIG. 1. In the example shown in FIG. 1, multiple slices S1, S2 and S3 are excited simultaneously, resulting in each slice generating an echo train of magnetic resonance signals, which are acquired according to the known blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration) technique (also called blipped CAIPI below). Details of such techniques are described, for example, in Setsompop et al., “Blipped-Controlled Aliasing in Parallel Imaging for Simultaneous Multislice Echo Planar Imaging With Reduced g-Factor Penalty,” Magnetic Resonance in Medicine, Vol. 67, pp. 1210-1224 (2012) and Setsompop et al., “Improving Diffusion MRI Using Simultaneous Multi-Slice Echo Planar Imaging,” NeuroImage, Vol. 63, pp. 569-580 (2012) and Cauley et al., “Interslice Leakage Artifact Reduction Technique for Simultaneous Multislice Acquisitions,” Magnetic Resonance in Medicine, Vol. 72, pp. 93-102 (2014).
A further type of magnetic resonance imaging is known as diffusion imaging, or diffusion-weighted imaging. This imaging technique is based on the fact that, due to their thermal energy, water molecules in tissue exhibit continuous random motion, known as Brownian motion. Water molecules contain hydrogen atoms, which are the most common atoms that are excited to resonance in magnetic resonance imaging. Because the excited water spins exhibiting this motion will encounter components of their cellular environment that exhibit different concentrations in different regions, the excited water spins will spread (diffuse) at different rates in different directions. In particular, the cell membranes restrict such diffusion. The basis of diffusion imaging is to activate magnetic field gradients in pairs so as to encode the spatial motion of the molecules. The signal intensity S in diffusion imaging is represented by the following equation:S=S0·exp(−b·ADC)wherein ADC is the apparent diffusion coefficient and b is the gradient factor, commonly referred to as the b factor. This factor represents a “summary” of the effect of the performance in diffusion gradients. The sensitivity to diffusion-based contrast is primarily dependent on the b factor value. So is the signal intensity when no diffusion gradients are present.
Diffusion-weighted echo planar images acquired with a twice refocused scheme (called bipolar encoding) with high b factor values suffer from a slice-position dependent signal dephasing. This dephasing is caused by additional, unintended magnetic field gradients, which occur as a consequence of the Maxwell equations, these fields being referred to as concomitant fields, having Maxwell terms. In a first order approximation, the effect of such Maxwell terms exhibits a quadratic dependency on the gradient amplitude and the spatial distance from the isocenter of the basic field magnet in the data acquisition scanner, i.e., it is inversely proportional to the basic magnetic field. Details of the reasons for and the effect of such fields are described in Meier et al., “Concomitant Field Terms for Asymmetric Gradient Coils: Consequences for Diffusion, Flow and Echo-Planar Imaging,” Magnetic Resonance in Medicine, Vol. 60, pp. 128-134 (2008).
The effect of such concomitant fields is more prominent at low field strengths of the basic magnetic field, such as 1.5 T, with high diffusion gradient amplitudes (i.e., high b factor values) and slices that are far off-center. For acquisitions using the Stejskal-Tanner diffusion scheme (called monopolar encoding), the concomitant fields from the intentionally-present diffusion gradients primarily lead to a deviation from the intended gradient amplitude, and thus from the intended b factor value. For bipolar acquisitions, however, this effect causes a signal dephasing, because the opposite gradient lobes are no longer balanced, as shown in FIG. 2 herein, which is FIG. 2 from the above-mentioned Meier et al. article. The conventional solution to this problem is to calculate slice-specific and gradient-specific correction moments, based on the equations presented in the aforementioned article by Meier et al. These calculated moments are then added to the bipolar diffusion-encoding gradients in order to compensate for the Maxwell terms.
In simultaneous multislice (SMS) acquisitions, such as the aforementioned blipped CAIPI, two or more slices are excited simultaneously by a linear combination of the respective slice-specific pulses, and are acquired as a collapsed dataset (i.e., data from all simultaneously excited slices), which can be separated into respective slices again in a separate post-processing step, such as by using slice GRAPPA, as noted above with regard to FIG. 1. For higher acceleration factors S (i.e., more simultaneously excited slices), and coils with few coil elements, it is desirable to maximize the spacing between the simultaneously acquired slices, in order to reduce the g-factor penalty.
The g-factor penalty results from the fact that the individual signals in SMS imaging are superimposed, the signals being received from respective individual coils of the MR data acquisition scanner. These coils necessarily individually occupy different spatial positions, but are in close enough proximity to each other so that the same nuclear spins will be detected by more than one of the multiple coils. Because each coil is situated at a different position in space, however, the effect of each individual coil on the reception of a given individual nuclear spin will be slightly different, and must be taken into account. This is done by calculating the aforementioned g-factor (geometry factor) for the coil array that is used. The extent to which the g-factor degrades the resulting reconstructed image is called the g-factor penalty. As noted above, in order to reduce the g-factor in SMS imaging, interslice image shifts are deliberately induced during the readout in blipped CAIPI, either by gradient blips on the slice axis or by modulating the phase of the RF pulses. After the data have been acquired, the simultaneously excited slices are collapsed into a single slice for entry of the data into k-space. The individual slices can be separated in the post processing, using the aforementioned slice GRAPPA technique as schematically illustrated in FIG. 1.
For SMS acquisitions, the aforementioned Maxwell compensation technique is not applicable, because the calculated correction moments are slice position-specific, and thus cannot be applied at the same time to two slices having a large spatial separation therebetween. This can lead to signal degradation, especially for slices acquired far off-center with high b-values at low magnetic fields. A correction method is described in United States Patent Application Publication No. 2013/0285656 wherein the respective pulses for the slices to be acquired simultaneously are shifted in time before the linear combination. The slice gradient then exhibits different asymmetries for the respective slice pulses. These non-balance slice-selection gradient moments are expected to provide a similar compensation behavior as the aforementioned method for the single-slice variant. This method, however, requires calculating a different combined pulse profile for each set of slice positions, which may lead to different acquisition times per slice set.